Growth hormone (GH) is one of the most important factors-controlling skeletal growth and development, but despite decades of work, the cellular targets and mechanisms of action of GH in bone remain unclear. The principal impediment to the study of GH action is the fact that GH stimulates IGF-1 production making it difficult to distinguish actions due to directly to GH versus those resulting indirectly through IGF-1. Historically, two theories have been developed to explain the biological significance of GH/IGF-1 interactions. The "somatomedin hypothesis" proposed that GH exerts it actions by stimulating peripheral (liver and other tissues) IGF-1 production. Alternatively, the "dual effector theory" postulated that GH directly stimulates target cell differentiation and that GH induced IGF-1 then stimulated clonal expansion of the differentiated cell population. Recent studies suggest a third possibility, namely that GH and IGF-1 can act collaboratively in certain settings through mechanisms involving a physical interaction between their receptors. Advances in technologies to manipulate the mouse genome have for the first time made it possible to distinguish the effects of each of these growth factors in vivo in a normal physiological context. We have used these methods to begin to identify GH actions in osteoblasts of GH that are independent of IGF-1 and are likely to profoundly influence osteoblast function. In this grant proposal, we describe newr in vitro and in vivo studies to define the mechanisms of GH action in osteoblasts and to determine their relationship to IGF-1 action. In Specific Aim 1 we will primary osteoblasts expressing or deficient in the IGF-1 receptor to characterize GHR signaling components and explore the concept of GH/IGH-1 collaboration by comparing the effects of GH on osteoblast proliferation, matrix deposition and differentiated gene expression. We will also examine the effects of GH administration in vivo to mice following conditional disruption of the IGF-1 R and compare the skeletal effects in this model to that observed in mice with an intact IGF-1. In Specific Aim 2, We will analyze the effect of conditional disruption of the GHR on IGF-1 R signaling, osteoblast proliferation, matrix deposition and differentiation in primary calvarial osteoblasts in vitro. Specifically, we will determine how signaling through each receptor and functional activity (e.g. proliferation, apoptosis) is affected by the presence of the other. We will determine the effect of Cre mediated disruption of the GHR from osteoblasts in vivo. Static and dynamic histomorphometric indices of cortical and trabecular bone and CT measurements will be used to compare the bone phenotype in the osteoblast GHR knockout mice to those seen in control mice at selected times during postnatal development. Finally, we will examine further the interaction between GH and IGF-1 in vivo by comparing the bone phenotype of mice lacking both GHR and IGF-1 R in osteoblasts. Completion of these aims should enable us to determine for the first time the mode of GH action in bone. A better understanding of the mechanisms of action of GH will have important implications for the action of this hormone in other tissues and for its use in the clinic. [unreadable] [unreadable]